The subject invention relates to thermoplastic molded parts having a new and improved stress reducing juncture. More specifically, thermoplastic molded parts are disclosed including a stress reducing juncture, defined by an elongated trough-like recess, which is operative to reduce differential shrinkage of the molded plastic, which would otherwise develop due to the uneven cooling of the plastic to room temperature. By this arrangement internal stresses are decreased thereby inhibiting the warping of the part.
In the prior art, many plastic parts are produced by injection molding techniques. In injection molding a two-part die is manufactured consisting of a core and cavity, which when assembled, defines an internal recess corresponding to the dimensions of the plastic part to be produced. Molded plastic material, such as mineral or glass filled thermoplastic polyester, is injected under pressure into the internal recess of the mold through channels or gates provided in the mold. The molten thermoplastic material fills the recess in the mold thereby defining the shape of the part. After the injection step is completed, the core and cavity sections of the mold are separated such that the molded plastic part can be removed.
Once the part has been molded, the heated thermoplastic will cool to room temperature. The rate of cooling and corresponding shrinkage of any particular plastic is dependent, in part, upon its local thickness as defined by the dimensions of the part. As can be appreciated, most thermoplastic parts will not cool and shrink uniformly since the local thickness throughout the part is not constant. For example, wherever there is juncture between a wall and another structural element, such as a second wall, flange or boss area, the local thickness of the plastic is increased. Thus, as the plastic cools to room temperature, shrinkage rates vary throughout the part depending upon the local thickeness of the part. This differential shrinkage, which is particularly acute at the corners or junctures of the part, is responsible for the development of internal stresses within the part. Internal stresses frequently cause the molded part to warp, which is unacceptable in most applications.
Accordingly, in the prior art a variety of methods have been utilized in an attempt to inhibit the unwanted warping of molded thermoplastic parts. For example, attempts have been made to vary the particular plastic utilized in the molding operation in an effort to minimize uneven cooling and shrinkage thereby inhibiting the build-up of internal stresses. This method has not proved particularly effective and in addition, the substitution of different plastic material is frequently undesirable. Another method utilized to minimize the warpage of parts includes the redesigning of the part in order to adjust the local thickness of the various structural elements. More specifically, attempts have been made to vary the local thicknesses of the part to achieve a more uniform cooling gradient. As can be appreciated, the redesigning of a part requires considerable trial and error experimentation and in addition may result in a part having dimensions which fail to meet the desired specifications.
A third method used in the prior art consists of varying the gating which is incorporated into the dies. More particularly, the positioning and dimensions of the gates or channels, which direct the molten thermoplastic material, from an injection molding machine, to the interior of the mold, can be varied in order to reduce warpage. However, a change in the gating, similar to the above described method of changing the type of plastic material, is not particularly effective.
Accordingly, it is an object of the subject invention to provide a new and improved thermoplastic part having a stress reducing juncture which functions to inhibit the warping of the part during the cooling of the plastic to room temperature.
It is a further object of the subject invention to provide a new and improved thermoplastic molded part wherein a stress reducing juncture, defined by an elongated trough-like recess, is provided in a wall of the part and is operative to reduce the differential shrinkage of the molded plastic which would otherwise develop due to the uneven cooling of the plastic to room temperature.
It is another object of the subject invention to provide a new and improved thermoplastic molded part having a stress reducing juncture for inhibiting the warping of the part, which is located at the intersection between wall members or other structural elements such as a flange or boss area.